1. Field of the Invention
The present invention relates to an ignition coil device for an internal-combustion engine which has an open magnetic circuit structure.
2. Description of Related Art
FIG. 8 is a sectional view showing a conventional ignition coil device for an internal-combustion engine, and FIG. 9 is an enlarged view of an essential section of FIG. 8. The ignition coil device for an internal-combustion engine shown in the drawings is provided in a plug hole 4 formed by a cylinder head 1 and a cylinder head cover 3; it has a spark plug 2 inserted at the distal end thereof.
The ignition coil device for an internal-combustion engine is provided with: a bottomed cylindrical case 5; a columnar core 6 which extends along the center axis of the case 5 and which is composed of multiple layers of silicon steel plate strips; a primary coil 8 provided around the outer periphery of the core 6; a secondary coil 9 provided around the outer periphery of the primary coil 8, magnets 7 attached to both ends of the core 6 to prevent magnetic fluxes, which are generated in the primary coil 8, from saturating the core 6; a control circuit unit 10 which incorporates a power transistor (not shown) for controlling the supply of power to the primary coil 8 and which is provided above the core 6; a connector 19 having a terminal 18 electrically connected to the control circuit unit 10; and an insulator 11 composed of a thermosetting epoxy resin charged into the space in the case 5.
The primary coil 8 has a bottomed cylindrical primary bobbin 12 and a primary winding 13 constituted by a conductor wound around the primary bobbin 12. The secondary coil 9 has a bottomed cylindrical secondary bobbin 14 and a secondary winding 15 constituted by a conductor wound around the secondary bobbin 14.
In the aforesaid ignition coil device for an internal-combustion engine, the primary current supplied to the primary winding 13 of the primary coil 8 is interrupted by a signal from the control circuit unit 10 causing high voltage to be generated at the secondary winding 15. The high voltage is led to the spark plug 2 via a high-voltage terminal 16 and a spring 17 to cause a spark discharge at a gap 20.
In the conventional ignition coil device for an internal-combustion engine, the primary coil 8, the secondary coil 9, the core 6, and the control circuit unit 10 are disposed in the case 5, and the insulator 11, which is cured later, is charged into the case 5. After this, the insulator 11 is cured at a high temperature of about 130 degrees Celsius so as to secure the primary coil 8, the secondary coil 9, the core 6, and the control circuit unit 10 in the case 5. Hence, for example, during natural cooling after the heat curing, the force generated by the shrinkage of the insulator 11 is undesirably applied to the core 6 due to the difference in thermal expansion coefficients between the insulator 11 and the core 6, thus causing the core 6 to become distorted. This results in an increase of core loss which deteriorates the magnetic properties, and presents the problem of a lower output voltage of the ignition coil device for an internal-combustion engine.
Further, during the operation of a vehicle which incorporates this ignition coil device, the primary current flows into the primary winding 13, and the ignition coil device becomes heated due to the heat generated by the primary coil 8, the secondary coil 9, et., whereas it is subjected to natural cooling when operation is stopped. Thus, as the vehicle is repeatedly started and stopped, heat stress occurs mainly in the core 6, the insulator 11 placed between the core 6 and the primary coil 8, and the primary bobbin 12. As a result, cracks develop in components such as the insulator 11 and the primary bobbin 12, which in turn causes internal leakage and creates a problem in that, in the worst case, no output voltage is generated.